He decided not to shoot again
Perhaps Chewie did spare Kylo. Whether or not he intentionally botched the first shot, once Kylo was on his knees in pain on the catwalk, Chewie could have fired another bolt to finish the job, but didn't.
It's true that the first shot provoked instant return fire from the nearby stormtroopers. From the novelization:
Hostile fire being something the group of stormtroopers could react to without having to wait for an order, they immediately blasted back at Chewie.
This may have prevented Chewie from taking another shot, but Chewie has been in worse situations and I feel he could have had a second shot if he wanted one, especially given Kylo's very vulnerable position.
Peter Mayhew tweet
There are no direct official statements on the matter.
However, after the comic was published, Chewbacca actor Peter Mayhew replied to it on Twitter:
In case it hadn't crossed your mind. @starwars @TheWookieeRoars @KyloR3n #TheForceAwakens pic.twitter.com/8wjHeD6h0l
Peter Mayhew (@TheWookieeRoars) January 9, 2016
While not a direct confirmation of Tyson Murphy's interpretation of the scene, it does suggest that Mayhew had given it prior thought and it may have been at least a point of discussion during the filming of the scene.
Apart from these observations, there is no other evidence that he held back.